In Brochos 30a the Gemora discusses the nusach to be used in Tefilas Haderech.
The Gemora first suggests Tefilas Haderech should be said in loshon yochid - שתוליכני לשלום . Abaye then says we should say Tefilas Haderech in loshon rabim - שתוליכנו לשלום. Rashi explains that by partnering our tefilla with others and davening collectively, we have greater zechusim and our tefilla will be heard. Indeed most of our tefillos are in loshon rabim, e.g. Shmone Esrei, every brocha we make - אלקינו etc.

Fresh from all of the great Daf Siyumim around the world, we are familiar that in a Siyum Mesechte we also say in loshon rabim: ... מודים אנחנו לפניך ... ששמת חלקנו מיושבי בית המדרש. We join with all others who are learning and being mesayem. This is the great Koach of the Rabim, which we B'H invoke each day when we join the rest of Klal Yisroel all learning the same daf.

It is interesting to note, however, in the Siyum of a Seder Mishnayes, the common printed version of the Hadran has these same words, but in loshon yochid: ... מודה אני לפניך ... ששמת חלקי מיושבי בית המדרש.

What is the reason for this difference between a siyum on Gemora or Mishnayes?
Should the tefilla really be the same in loshon rabim, but the printers of the Gemora and Mishnayes have introduced this difference?